Field of the Invention
The present invention relates to a method of manufacture for insensitive crystalline high explosive (HE) molding powders, and more particularly, to such a method wherein the crystalline HE within the molding powders are nanocrystalline and are uniformly coated with non-energetic, i.e. inert, binders.
Related Art
Explosive molding powders are known in the art and are used for various types of ordnances such as: grenades, land mines, missile warheads, and demolition explosives. Such explosive molding powders are extrudable or pressable into a desired shape for use in a given ordnance system. Common high explosive (HE) materials used in such explosive molding powders include HMX, RDX, and CL-20. Such an HE material is mixed with a binder, to bind the crystalline particles together, so that the resulting explosive powder can be physically molded to meet the particular application requirements. The primary usage of the binder beyond the “binding” functionality is to make the explosive material less sensitive to external stimuli.
Often, explosives applications involve balancing the desired insensitivity of the explosive with its performance—especially in applications involving boosters. Current booster explosives must have a sufficient energy output to reliably initiate newer, relatively insensitive main charge explosive fills—while the booster itself desirably has a lower level of sensitivity to unintended stimuli. Most existing booster HE formulations exhibit undesired levels of sensitivity; thereby, increasing the vulnerability of the entire munition to accidental initiation.
The crystal size of an HE can influence sensitivity to unintended stimuli, such as shock and impact; more specifically, it has been demonstrated that the sensitivity of a high explosive decreases with decreasing crystal size. See, Stepanov et al. “Production and Sensitivity Evaluation of Nanocrystalline RDX-based Explosive Compositions”, Propellants, Explosives, Pyrotechnics, v. 36, 2011. Further, improved performance characteristics are also associated with crystal size reduction. For example, the detonation failure diameter, also referred to as the critical diameter, is known to decrease with decreasing crystal size. However, while Fluid Energy Milled (FEM) HMX is available with typically an average mean diameter of several microns, nanocrystalline HEs are currently not commercially available; and, prior to this disclosure, there was no known commercial method of production thereof. Further, the FEM HMX that is currently commercially available is not as insensitive as desired and does not provide the needed performance in small critical diameter applications.
U.S. Pat. No. 6,485,587, issued Nov. 26, 2002 to Han et al., incorporated herein by reference, discloses methods for the preparation of explosive molding powders typically consist of batch slurry coating of crystalline HE with a binder. In such processes, the explosive crystals are dispersed in aqueous slurry, to which a lacquer solution consisting of an organic solvent and the binder ingredients are added. However, dispersion of nano-crystals in aqueous slurry is not effective due to the high tendency of such very small crystals to agglomerate, resulting in poor binder coating about the crystals. Further, there is a tendency of nanocrystals to “ripen” (Ostwald ripening)—resulting in a detrimental increase in the mean crystal size.
Bead mills have been used to create nanosized HE materials in the past; however, the material is trapped in an aqueous solution. An efficient method of filtering out the material, or coating it directly, without ripening the explosive, has, up to this point, not been found. See, Patel et al “Production and Coating of Nano-RDX Using Wet Milling,” National Defense Industrial Association Insensitive Munitions and Energetic Materials (NDIA IM/EM) Symposium Proceedings, 2007.
Considering the above facts, there is a need in the art for a more insensitive HE material; with enhanced performance characteristics, especially in small critical diameter applications; that is, manufactured in an effective, safe, and relatively economical way.